The present invention relates to an improved covering film for casting process by vacuum sealed molding, and more particularly relates to an improvement in the composition of a covering film used for casting process by vacuum sealed molding.
The so-called casting process by vacuum sealed molding starts with preparation of a mold utilizing solidification of a refractory filler building up the mold by vacuum suction. At the starting of the molding operation, a heated thermoplastic resin film is brought into a snug surface contact with the pattern surface of an original pattern by application of vacuum suction. After mounting a corridor-type molding box in position on the original pattern, the refractory filler is charged into the center cavity of the molding box. When the center cavity is replete with the refractory filler, the top opening is closed by a separate sealing film and vacuum suction is applied to the refractory filled via the molding box in order to cause provisional solidification of the refractory filler. After the solidification is complete, the vacuum suction applied to the original pattern is cancelled and the latter is separated in order to obtain one mold half. In a substantially similar manner, the other mold half is obtained. The mold halves are firmly coupled to each other in order to obtain a complete mold having an internal mold cavity whose walls are tightly covered by the covering films. Molten metal is charged into the mold cavity under application of the vacuum suction to the mold and, after complete solidification of the charged metal, the vacuum suction is cancelled in order to disassemble the mold and obtain a cast product.
As the extremely high temperature molten metal is charged into the mold cavity in the above-described process, the covering films on the wall of the mold cavity melt and penetrate into the refractory filler due to the vacuum suction applied to the mold. In other words, the covering film disappears from the walls of the mold cavity which are then covered by the molten metal charged into the mold cavity.
In the case of a large mold cavity, the molten metal coming into the mold cavity first spreads over the bottom wall of the lower mold half and gradually increases in depth. That is, it takes rather a long period before the top of the molten metal reaches the top wall of the upper mold half. Due to this time lag, the covering film on the top wall of the upper mold half disappears due to the radiative heat from the rising top surface of the molten metal. This disappearance of the covering film tends to cause destruction of the upper mold half as the refractory filler is directly exposed in the mold cavity before contact with the molten metal charged into the mold cavity. This direct exposure of the refractory filler may take place on the side of the lower mold half also. Depending on the extent of the vacuum suction, granular size of the refractory filler and spreading speed of the molten metal, the covering film portions in the vicinity of the leading ends of the spreading molten metal melt and are sucked into the refractory filler so that the refractory filler is exposed in the mold cavity. Such exposure is liable to cause casting defects such as sand inclusion.
As the molten covering film is sucked into the refractory filler, the molten thermoplastic synthetic resin forms a shell layer together with the refractory filler. However, since the penetration speed of the molten resin is too large, the molten resin is dispersed over a wire area in the refractory filler at relatively low density. Such a low density of the dispersed molten resin naturally results in low bonding strength and low mechanical strength of the shell layer, thereby causing undesirable sand inclusion and texture degradation at charging of the molten metal.
As a solution to this problem, a new process has already been proposed by the U.S. Pat. No. 4,043,376. In the case of this proposed new process, the thermoplastic resin covering film sucked onto the original pattern is coated with a solution of an initial condensate of a thermositting resin to a thickness, preferably, in a range from 2 to 100 micronmeters in the solid state. After the coating, a molding box is mounted in position on the original pattern. The subsequent operations are substantially similar to those of the above-described conventional casting process.
When molten metal is charged into the mold cavity of the mold prepared by the above-mentioned new process, the initial condensate of the thermosetting resin located between the covering film and the refractory filler melts first due to heat of the charged molten metal and penetrates into the surface portion of the refractory filler near the mold cavity. This penetration of the thermosetting resin initial condensate forms a hardened layer in the above-described surface portion of the refractory filler and presence of such a hardened layer effectively prevents mold destruction and/or sand inclusion despite of advanced disappearance of the covering film on the cavity wall.
From the view point of reduction in casting defects, the merit of the above-described new process should be highly appreciated. However, it is inevitably accompanied by an undesirable operational complication in the molding process. That is, in order to assure appreciable covering characteristics, it is necessary to form a layer of the thermosetting resin initial condensate to a thickness, preferably, in a range from 2 to 100 micronmeters by solution coating. This solution coating is generally practiced by manual operation while using tools such as spray guns after the covering film is sucked onto the original pattern. Such a manual operation is particularly needed when the pattern surface of the original pattern is highly complicated in shape.
As is well known, spray of such a solution is unwelcome for the operators from the view point of labor hygienics. In addition, use of the refractory filler in the molding process is liable to generate powdery dust which is unsuited for manual operation. For these reasons, it is preferable to reduce manual operational steps in the vacuum sealed molding as much as possible and introduce mechanization of the whole process. Thus, requirement for the manual operation in the above-described coating step forms a great bar to streamlining the whole process by mechanization and automation of the operational steps. Solution coating by manual operation also tends to result in uneveness in thickness of the coated layer. Further, very complicated shapes of the pattern surface makes it difficult to carry out the coating operation even by spray guns.
It was confirmed by the inventors of the present invention that a certain type of thermoplastic resin produces not only a gas or gases after decomposition by high temperature heating but also dark brown burnt dregs which can be called as a kind of initial carbide. It was confirmed also that a covering film can be made of a thermoplastic resin productive of such burnt dregs and a thermoplastic resin not productive of such burnt dregs and, when the content ratio of the two types of the thermoplastic resins is properly adjusted, the covering film can permeate moderately into the refractory filler at charging of molten metal into the mold and form a relatively thin shell layer near the surface portion of the refractory filler defining the mold cavity, thereby successfully avoiding production of casting defects such as mold destruction and sand inclusion even after the film has disappeared from the surface of the refractory filler.
It was further confirmed that the above-described effects can be expected when the above-described mixed composition exists in the thickness direction of the covering film. Consequently, such a mixed composition may be made in the form of either a single-layered composite film including a thermoplastic resin productive of burnt dregs and a thermoplastic resin not productive of burnt dregs or a double-layered films one of which is made of a thermoplastic resin productive of burnt dregs and the other of which is made of a thermoplastic resin not productive of burnt dregs.